1. Field of the Invention
The present invention relates to an X-ray fluoroscopic apparatus for limiting an entrance exposure rate representing an X-ray quantity per unit time radiated onto a patient.
2. Description of the Related Art
With an X-ray fluoroscopic apparatus, X-rays are radiated onto a patient, an image obtained from the transmitted X-rays is converted into an optical image by an image intensifier tube, the optical image is picked up by a TV camera and displayed on a TV monitor, and a diagnosis is made based on the displayed image.
In an X-ray fluoroscopic apparatus of this type, the X-ray quantity radiated onto a patient per unit time, i.e., the entrance exposure rate must be limited in view of patient's safety. In the H.H.S standards of the United States, the entrance exposure rate must be 10 R/sec. if the X-ray fluoroscopic apparatus has an automatic entrance exposure rate limiter, and 5 R/sec. if otherwise. These values are determined in view of the diagnostic resolving power and the safety of the patient.
When the entrance X-ray quantity is constant, the X-ray quantity transmitted through the patient varies depending on the patient's body thickness. However, the values cited above are not determined considering the patient's body thickness. When the patient's body thickness is large, the transmitted X-ray quantity is decreased, which decreases the brightness of the image displayed on the monitor, thereby resulting in a decrease in diagnostic precision.